Most young "First Defenders" will either rush in too fast (e.g., jump at the ball), or play too "Soft". If they rush the First Attacker (the Onball Attacker) too hard, he can blow past them -- but if they are too "soft", they don't slow down the First Attacker and the First Attacker can easily make a pass or take a shot. SO, you want to teach your players to "Pressure" the ball, but not too hard and not too soft. The Premium "Win the 50/50 Ball or Be the First Defender 1v1 Attacking and Defending" Practice Game is a good way to teach this. It teaches players that if they rush in and miss the tackle, or if they play too "soft", there is a risk of giving up a score (in the game it could cost them 3 points), whereas if they can "contain", "steer" (also called "Dictating" or "Channeling") and slow down the attacker they will earn 1 point for themselves. As to why defenders of all ages must be careful about pressing too hard, an analogy would be to a U.S. football cornerback covering a receiver, or a linebacker trying to make an open field tackle -- they must be careful not to rush in because the attacker might get past them and score. There is a very good Video Clip of proper 1v1 defense at the "Training Girls & Women to Win" video clips. As players get older and if they play Select or High School, their teammates are supposed to let a defender know if they have "Support" of a Second Defender (i.e., that they have "back-up") by saying something like "I'm here" so the First Defender knows it's safe to try to tackle, but that normally doesn't happen at young ages or in Rec. Here's a link to the "Win the 50/50 Ball 1v1 Attacking and Defending Game"Here's a link to an article about Defensive Containment that discusses, Jockeying (Shepherding), Defensive Footwork, Marking, Channeling: www.soccerhelp.com/premium/Marking.shtml
Here's the definition of "Dictating" from the Premium Dictionary (also called Steering or Channeling):
(aka "Channeling", "Dictating" or "Steer"). The First Defender should angle his body and position himself to "Dictate" the "onball attacker" toward the sideline. This is the same concept as "channeling" the ballhandler. If the ballhandler is on either the left or right side of the field (anywhere except the exact center), the First Defender should "Dictate" the movement of the ballhandler by running toward the ballhandler in a slightly curved direction favoring the center of the field, so the ballhandler is discouraged from trying to move to the center of the field, and instead is encouraged to move toward the sideline (which is where the Defender is making it easier for the ballhandler to go). This is a desirable tactic because it poses much less risk to the defending team than if the ballhandler is able to turn into the center of the field. The Defender should favor the center and turn his body so the ballhandler cannot easily get past him to the center of the field. This encourages the ballhandler to go toward the sideline. Forcing the ballhandler toward the sideline reduces the passing options available to the ballhandler and in the Attacking Third reduces the risk of the ballhandler getting a clean shot on the front of the goal (i.e., if he gets off a shot from the sideline area he has a bad angle). (See "Defense" and "Marking").There are 3 great free Video Clips on SoccerHelp of how to teach 1v1 Defending. Unless you are an expert at this, I strongly recommend you watch them. They will show you how to properly teach 1v1 defending, which is VERY important and is easy to teach. If you watch all 3 you will see how to Pressure, Control, Dictate ("Channel"), Tackle and Deny the Turn. They show the proper technique for Jockeying and good defensive foot work and body position.
I recommend you watch them in the order below, because it will be more understandable for you if you do.
Video Clip of 1v1 Defending https://soccerhelp.com/shshop/ans-deny-turn.php